Digital printing of textile substrates involves applying small quantities of colorant (e.g., inks, dyes, pigments, etc.), known as pixels, in predetermined areas of a textile substrate, for example via ink jet printing. Typically, only one colorant is used for a particular pixel, and variations in colors and shades are accomplished by positioning different colored pixels in adjacent or near-by areas. Although the actual color of an individual pixel is not changed, the impression to a viewer is that the area containing different colored pixels is a color or shade that is different than any of the individual pixels in the associated area. The impression is created because the pixels are so small that a viewer cannot readily perceive the individual pixels. Rather, the viewer perceives an average of the pixel colors.
Conventional digital printing of polyester and polyester-rich fabrics is typically an indirect printing process. Disperse dyes are first printed on coated transfer print paper and then the print is sublistatically transferred to a textile substrate using a heated press or similar machine. Printing technology has evolved to allow fabrics to be printed directly, and inks and dyes are available for directly printing virtually any type of woven or non-woven fabric, including cotton, polyester-cotton, silk, and nylon. Inks containing acid dyes also are available for printing polyamides such as nylon, silk and wool. Inks composed of dispersions of disperse dyes are available for direct printing of 100% polyester fabrics and polyester-rich fabrics.
Direct digital printing has several advantages over transfer (sublistatic) printing. For example, there are a wider selection of inks and dyes available that have improved wash and light fastness characteristics and increased size of the color gamut. Digital printing inks and dyes typically have less negative effects on textile properties such as hand, for example. In addition, because of the wider selection of inks and dyes, color matching is enhanced. Digital printing may also help streamline fabric production processes.
However, direct digital printing on textiles that vary in fiber content, weight, thickness, ink absorbency, and yarn size, that must be washable, light fast, crock resistant and wearable and require multiple ink sets can present a broad array of challenges. For example, a colorant may bleed outside of the intended pixel area, or may be absorbed into the fibers of a textile substrate. If a colorant does not completely fill one or more intended pixel areas, an image on a textile substrate can lose color intensity due to the underlying textile substrate color. If a colorant is absorbed into the textile fibers, color intensity can also be lost. If a colorant bleeds outside of the intended pixel area, image sharpness and intensity can be negatively affected.
In direct digital printing, evenness of print in fully covered (blotch) areas can be difficult to obtain as can be fine line definition. In traditional fabric printing techniques, e.g., rotary screen printing, ink chemistry controls print quality. For example, an ink may contain materials such as alginates or synthetic thickeners which, when combined with dyes and other additives, and in combination with screen selection, will provide both even blotches (larger areas covered with print) and fine line definition. Additionally, in rotary screen printing it is possible to tailor each color, (i.e., screen) to meet specific performance needs. For example, if a blotch color is being printed on one or several screens, it may have a different paste formulation than a screen being used to print a fine line. This is not possible with digital printing since both fine lines and blotches are printed with the same inks. Additionally, if a digital print head is not functioning perfectly, there may be striations in the blotch areas.
To address some of the problems associated with direct digital printing, traditional print thickeners such as sodium alginates, have been added to digital printing colorants. Unfortunately, the use of traditional print thickeners in direct digital printing processes can be problematic. Alginates absorb moisture and must be removed before shipping due to textile substrate stiffness and odor. Also, these thickeners must be applied to a textile substrate via a coating, printing or pad-dip application. If viscosity becomes high, application via pad-dip can be difficult to control. Application via a coating mechanism, either direct such as knife over roll or knife over gap or foam coating, gives better control but may be more expensive than pad-dip methods.
Textile fabrics have been treated with fluorochemicals to lower the textile substrate surface tension. This results in aqueous-based inks sitting on the textile substrate surface and then being allowed to fix to the fibers. Unfortunately, drying can be very slow and ink droplets can, if the amount of fluorochemical is too high or uneven, literally run off the textile substrate.
In digital printing, color yield (i.e., the amount of ink required to color a particular portion of a fabric) is an important issue because ink costs may be more than one hundred times that of traditional dyes. In fact, ink costs may well represent over 30% of the cost of a digitally printed fabric.
Traditionally, a printed fabric, following printing, is fixed in a steamer or by a thermofixation method, washed and then dried. Washing is required to remove any unfixed dye and to remove print paste residuals. If left on a printed fabric, unfixed dye and print paste residuals can cause problems with crocking, wash fastness and staining, hand, and flammability. If a printed fabric is intended to meet flame resistance criteria, print paste residuals may actually increase the fabric's propensity to burn.
Accordingly, there exists a need for improved methods of digitally printing textile substrates.